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Dielectric resonator based multiport antenna system with multi-diversity and built-in decoupling mechanism
Int. J. Electron. Commun. (AEÜ) 119 (2020) 153193
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International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Regular paper
Dielectric resonator based multiport antenna system with multi-diversity and built-in decoupling mechanism Gourab Das, Nikesh Kumar Sahu, Ravi Kumar Gangwar ⇑ Department of Electronics Engineering, Indian Institute of Technology (ISM) Dhanbad, Jharkhand 826004, India
a r t i c l e
i n f o
Article history: Received 19 February 2020 Accepted 27 March 2020
Keywords: Multiple-input-multiple-output (MIMO) Dielectric resonator antenna (DRA) Diversity techniques Envelope correlation coefficient (ECC)
a b s t r a c t In this article, an eight-element, sixteen port dielectric resonator (DR) based double sided multi-port antenna is presented, which supports the unique dual-directional pattern diversity feature. The orientation of the double-sided dielectric resonator antenna (DRA) is one of the novel features of the proposed antenna. Four cylindrical dielectric resonator antennas (cDRAs) are placed at the top of the substrate, and each top cDRA is excited with the help of two orthogonally placed coplanar waveguide (CPW) fed conformal microstrip lines. In the same manner, the other four cDRAs are placed at the bottom of the substrate, and each cDRA is fed via conformal strip lines. The proposed antenna works in between 5.4 GHz and 6.0 GHz. Several isolation techniques (polarization diversity, pattern diversity, and separation between the antenna elements) within the antenna elements are approached to improve the antenna performance. As a result, without using any isolation improvement technique, the proposed structure achieves 17 dB of isolation between the ports, which makes the proposed radiator unique. The similarity in simulated and experimental outcomes confirms that the designed 16-port multi-port antenna system is suitable for Wireless LAN applications. Ó 2020 Elsevier GmbH. All rights reserved.
1. Introduction Modern cellular communication systems require high data rates with improved system capacity and reliability. This requirement enforces antenna engineers to design and develop multi-port antenna systems. Multi-port antenna systems are designed by placing more than one radiator on a single substrate. Multiport MIMO technology generally uses diversity techniques to improve the reliability of any communication system by mitigating the multipath fading problem. To improve spectral efficiency, the main aim is to spread the total transmitted power over the radiators. Thus, MIMO antennas contribute to the enhancement of the reliability and data rate of the communication system without losing an additional transmitted spectrum or power [1]. In MIMO based radiating structure, the correlation between the fields obtained from multiple ports should be minimum. But, it is problematic to adjust many radiators in a single antenna system with maintaining high isolation since the radiation pattern of different radiators can affect each other as well as the MIMO channel characteristics. To improve the port isolation level, one can use
⇑ Corresponding author. E-mail addresses: [email protected] (G. Das), nikesh.sahu100@gmail. com (N.K. Sahu), [email protected] (R.K. Gangwar). https://doi.org/10.1016/j.aeue.2020.153193 1434-8411/Ó 2020 Elsevier GmbH. All rights reserved.
different isolation techniques such as reflectors/defected ground structure (DGS)/parasitic elements, but it is at the cost of radiation characteristics [2]. So, isolation improvement without any extra decoupling structure gives good matching efficiencies and radiation characteristics. On the other hand, dielectric antennas have gained extensive recognition in cellular communication, since it offers numerous advantages like high radiation efficiency, no conductor loss, improved gain, ease of excitation among others [3]. Therefore, at microwave and millimeter-wave frequencies (metallic losses are very high), dielectric based radiators are widely used [4–9]. In [4], the design and analysis of 8-port based MIMO cDRA for 2.4/5.8 GHz WLAN application were explained. For beam tilting, a reflector was used to achieve pattern diversity. The radiator, as mentioned above, takes over a huge volume of 160 160 14.8 mm3. A rectangular DRA based three ports MIMO antenna with a dimension of 56.6 31.5 14.09 mm3 was described in [5]. The triple port produced three decoupled degenerate modes in the DRA to achieve improved isolation between the ports. Probe fed based six-port two-element MIMO DRA was discussed in [6]. Each DRA was excited by three different ports. The occupied area of the proposed antenna was 100 80 13.56 mm3. It used spatial and polarization diversity to achieve low mutual coupling between the ports. An annular-shaped cDRA supporting dual-port pattern
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diversity was presented in [7]. The pattern diversity was accomplished by stimulating the two degenerate orthogonal modes (TE01d and TM01d) in the annular cDRA. The size of the realized radiator was 100 100 17.62 mm3. In [8], a dual-port rectangular DRA with a dimension of 80 80 7.6 mm3 was proposed for 2.6 GHz LTE band. The isolation was improved through pattern diversity, by etching two rectangular slits from the ground plane. In [9], a dual-port DRA based MIMO antenna with an occupied area of 50 50 14.6 mm3 was discussed. Polarization diversity was achieved by stimulating orthogonal modes in the DRA. A compact back to back four-port MIMO DRA was discussed in [10]. The antenna used polarization and pattern diversity for isolation improvement between the ports. In literature, most of the DRA based multi-port radiators consisted of 2-ports. However, such type of antenna cannot improve the quality of the transmission link as the channel capacity has a linear relationship with the number of spatial streams generated by each antenna ports [11]. Due to these reasons, recently, some of the DRA based MIMO antennas have been reported, which consist of more than two ports [4–6]. However, most of these antennas are large or having poor MIMO performance. In this paper, a compact double-sided DRA based sixteen-port MIMO radiator is presented for wireless access point applications. The compactness is achieved by placing back to back DRA orientation since this type of arrangement provides effective utilization of the ground plane. Four cDRA are positioned at the top of the substrate and excite in the forward direction. Another four cDRA are placed at the lower side of the substrate and radiated in the backward direction. The antenna consists of sixteen ports since each cDRA is stimulated with the help of two ports.
Additionally, for the very first time, three distinct different isolation techniques i.e polarization diversity, pattern diversity, and separation between the DR, are used within a single antenna system for isolation improvement. As a result, this radiating structure attains low mutual coupling (better than 17 dB) within the operating band without using any extra structure. Also, the proposed antenna provides two oppositely oriented radiation patterns in space that means higher chances of having independent channels, thus deliver higher channel capacities as well as low field correlation. The proposed radiator is easy to construct, and the total volume of the realized radiator is 90 80 14.6 mm3. The proposed antenna also analyzes how the combination of two or more diversity techniques at a time improves isolation in detail. Different MIMO performance parameters like envelope correlation coefficient (ECC), diversity gain (DG), mean effective gain (MEG), and channel capacity loss (CCL) are also analyzed. More significantly, the proposed MIMO antenna system described in this paper is a low profile for a commercially available wireless access point system, which enables fourth-generation communication systems [12]. Section-II and III of the presented paper show the geometry of the antenna and analysis of the proposed antenna, respectively. The results and discussion of the proposed antenna and conclusion are described in section-IV and V, respectively.
2. Formation of the proposed radiator The geometry of the projected 16-port dielectric resonatorbased radiator is displayed in Fig. 1. From Fig. 1(a), it can be perceived that four alumina ceramic-based cDRAs (DR-1, 3, 5 and 7)
Fig. 1. Geometry of the proposed MIMO antenna (a) Top-view (b) bottom-view; (c) Transparent side view; (d) side view of DR-1 and 2.
G. Das et al. / Int. J. Electron. Commun. (AEÜ) 119 (2020) 153193 Table 1 Optimized design parameters. Symbol
Dimension (mm)
Symbol
Dimension (mm)
LS WS Hs L1 = L2 W1 = W2 P1
90 80 1.6 8 2 0.25
H1 H2 LX LY DU = DL HU = HL
6.5 6.0 30 20 14 6.5
(edra = 9.8) are positioned at the top of the FR4 substrate (esub = 4.4). Each of the top cDRA is excited with the help of two orthogonally fed CPW fed conformal strip lines. On the other hand, another four ceramic material based cDRAs (DR-2, 4, 6 and 8) are placed at the bottom of the substrate. cDRA, located at the bottom of the substrate, is excited by two microstrip line fed conformal strip lines as shown in Fig. 1(b). To accommodate Ports-1, 5, 9 and 13, a portion of the substrate (LX LY) has been removed. As a result, the top of the substrate contains a total of eight ports. The top of the substrate consists of 4 cDRA with eight different ports; similarly, the bottom of the substrate contains 4- cDRAs along with 8- individual ports that make the proposed antenna as 16- port MIMO antenna. Different optimized design parameters are shown in Table 1. It should be noted that the length (LX and LY) of the rectangular removed portion is not equal since such configuration offers equal length for all the feedings and the connected ground plane between the different antenna elements. In the MIMO system, the connected ground plane is important to achieve improved diversity performance [13].
3. Analysis of the proposed radiator The design and analysis of the proposed radiating structure have been performed with the assistance of Ansys HFSS EM simulation software. Figs. 2 and 3 show the reflection coefficient of each port and isolation between the ports of the given multi-port radiator for various ports, respectively. Fig. 2 reveals that all the ports can able to cover the band 5.8 GHz WLAN (5.725–5.875 GHz) band. There is a minor difference between the reflection coefficients which occur due to the utilization of two different feeding mechanisms [14–15]. Fig. 3 shows the isolation variation for ports-2 to 16 with respect to port-1. From Fig. 3, it can be observed that the isolation between the ports is better than 15 dB for all the cases. In the MIMO antenna system, the isolation is achieved between the ports by imposing different diversity techniques. In the proposed MIMO
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antenna, three different isolation improvement techniques (polarization diversity, pattern diversity, and separation between the elements), as well as their combination, are used to achieve improved diversity between the ports. Different isolation improvement techniques between the ports are discussed below: 3.1. Polarization diversity Polarization diversity is established by exciting orthogonal mode in the cDRA. This approach is applied in DR-1 due to ports1 and 2. DR-1 supports dual hybrid modes i.e. HEx11d and HEy11d modes because of ports- 1 and 2, respectively. Since, HEx11d and HEy11d modes are decoupled orthogonal mode (via polarization difference), so polarization diversity plays a crucial role in achieving improved isolation between port-1 and 2. Fig. 4(a) shows E-field lines orientation in DR-1 due to port-1 and port-2. It reveals that orthogonal modes ðHEx11d and HEy11d ) are generated in DR-1. Here, HEx11d and HEy11d denotes variation of field lines along the X-axis and Y-axis, respectively. As a result, the proposed antenna achieves 17 dB of isolation between ports-1 and 2. 3.2. Pattern diversity Pattern diversity is achieved by exciting the cDRA in a different direction to realize a spatially de-correlated beam pattern. From Fig. 1(c), it can be perceived that DR-2 is positioned at the bottom side of the substrate, just below of the DR-1. DR-2 is excited by ports- 3 and 4. The exclusive feature of HE11d mode is that it always radiates in the broadside direction [16]. In this antenna structure, this elite feature of HE11d mode has been reserved in different directions with the assistance of different port to improve port as well as field isolation. Fig. 4(b) shows the 3D polar plot of the proposed antenna for Ports-1 and 3. Fig. 4(b) reveals that DR-1 is excited in the broadside (+Z) direction, whereas DR-2 is excited in opposite of broadside (-Z) direction with the help of port-3 and 4. So, DR-1 and DR-2 are excited in a complementary direction, which creates pattern diversity in two opposite directions. The radiation beam of DR-1 and DR-2 is separated spatially. As a result, we get improved isolation (S13) values (19 dB) between ports- (1, 3) in the working frequency range. So, the isolation between ports- (1, 3) has been achieved through pattern diversity. 3.3. Separation between the DR elements It is a well-known fact that mutual coupling among the antenna elements is high because of field interaction. So, this field interac-
Fig. 2. Reflection Coefficient of the proposed antenna (a) Reflection coefficient (Ports-1 to 8); (b) Reflection coefficient (Ports- 9 to 16).
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Fig. 3. Isolation response of the proposed MIMO antenna (a) Isolation variation (Ports-2 to 8 with respect to port-1) and (b) Isolation variation (Ports-9 to 16 with respect to port-1).
Port-1
Port-2
(a) Port-1
Port-3
(b) Fig. 4. E-field distribution and 3D-polar plot of the proposed antenna (a) E-field lines for Port-1 and port-2; (b) polar plot for port-1 and port-3.
tion can be minimized by creating a large spacing between them. Fig. 5 shows the minimum effective electrical path length between DR-1 and 3. It reveals that DR-1 and 3 are symmetric with respect to port-1 and port-6, and the spacing (D1) between port-1 and port-6 is 0.6k at 5.6 GHz. So, the spacing between DR elements plays an important role in isolation improvement between ports1 and 6. DR-7 is excited with the help of ports- 13 and 14. Fig. 5 also shows the effective distance between DR-1 and DR-7. Fig. 5 reveals that the effective electrical path length between DR-1 and DR-7 (D2) is also 0.85k at 5.6 GHz. The mutual coupling depends on the distance between the antenna elements. As the separation increases, the isolation level also improves [17,18]. So, the rectangular portion (LX LY), which is removed from the substrate, is used not only to accommodate the four ports. It also creates a large spacing between DR-1 and DR-7. This made the proposed antenna to achieve high isolation (better than 20 dB) between the port-1 and port-13 which is shown in Fig. 3(b).
3.4. Combination of different isolation techniques A combination of isolation improvement techniques means out of three isolation techniques, two or three isolation techniques are approached simultaneously. The advantage of the combined isolation improvement technique is that it can able to improve diversity performance significantly as compared to a single isolation improvement technique. Four cases of combination of isolation techniques are discussed here. They are given below: (i) Case-I (Combination of polarization and pattern diversity) (ii) Case-II (Combination of polarization diversity and separation between the cDRA) (iii) Case-III (Combination of pattern diversity and separation between the cDRA) (iv) Case-IV (Combination of polarization diversity, pattern diversity and separation between the cDRA)
G. Das et al. / Int. J. Electron. Commun. (AEÜ) 119 (2020) 153193
Fig. 5. Separation between DR (1 and 3) and DR (1 and 7);
(i) Case-I: DR-1 and DR-2 are placed at opposite sides of the substrate, as shown in Fig. 6(a). The direction of maximum radiation of DR-1 and 2 is also shown in Fig. 6(a). It can be observed that DR-1 is radiated in the forward direction due to port-1, and DR-2 is
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radiated in the backward direction due to port-4. So, pattern diversity exhibit between ports-1 and 4. On the other hand, polarization diversity is achieved, since orthogonal modes are generated in DR1 and DR-2 due to port-1 and port-4, respectively. As a result, the proposed antenna achieved more than 25 dB of isolation between these ports. Only polarization diversity (between ports-1 and 2) or pattern diversity (between ports-1 and 3) offers less than 20 dB of isolation between the ports, which we have already discussed earlier. But, the combination of diversity techniques offers 7–8 dB of isolation improvement between the ports. (ii) Case- II: It can be observed from Fig. 6(b) that ports-5 and 9 are placed orthogonally with respect to port-1. The direction of Efield in the respective ports is also shown in Fig. 6(b). It reveals that the orthogonal modal pattern is generated in DR-1 and 3 due to ports-1 and 5, respectively. Similarly, the orthogonality principle is also established in DR-1 and 5 with ports-1 and 9, respectively. So, polarization diversity (via polarization difference) plays an important role in improving isolation between ports- (1, 5) and ports- (1, 9), since decoupled orthogonal modes are generated in the cDRA which is shown in Fig. 6(b). Fig. 6(b) also shows the minimum electrical distance between DR- (1, 3) and DR- (1, 5). The minimum electrical distance (D1) between DR-1 and DR-3/5 is 0.6k at 5.6 GHz. So, polarization, as well as the separation between the cDR, plays a crucial role in achieving improved port isolation (21 dB) between ports- (1, 5) and ports- (1, 9). It can also be
Fig. 6. Combination of isolation techniques between the ports (a) polarization and pattern diversity; (b) polarization and separation between the DRA; (c) pattern and separation between the DRA and (d) polarization diversity, pattern diversity and separation between the DRA.
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observed in Fig. 6(b) that DR-1 and DR-7 are placed orthogonally with respect to port-1 and port-14. The minimum electrical path distance between DR-1 and DR-7 (D2) is 0.85k at 5.6 GHz. So, the combination of polarization diversity and separation between the antenna elements is utilized to achieve high isolation (better than 28 dB) between port-1 and port-14. The isolation level is better in the case of ports- (1, 14) than the ports- (1, 5) or ports- (1, 9), since the D2 value is high compared to D1. (iii) Case-III: It can be observed from Fig. 6(c) that DR-1 and DR(4, 6 and 8) are located at opposite sides of the substrate. The direction of radiation of these DRs is also shown in Fig. 6(c). It reveals that DR-1 is excited + Z direction for port-1 but, DR- 4, 6 and 8 are excited in –Z direction due to ports- 8, 12, and 15, respectively. So, their radiation patterns are separated spatially. Also, the distance (D1) between DR-1 and DR- (4, 6) is 0.6k at 5.6 GHz. On the other hand, the distance (D2) between DR-1 and DR-8 is 0.85k at 5.6 GHz. So, together pattern diversity and separation between the antenna elements provide high isolation (better than 25 dB) between ports- (1, 8), ports- (1, 12), and ports- (1, 15). (iv) Case-IV: It can be observed from Fig. 6(d) that ports- (7, 11 and 16) are positioned orthogonally with respect to port-1. So, polarization diversity exists in between them. Fig. 6(d) also shows that DR-1 excited in the forward direction (+Z-direction) with port1 but, DR- 4, 6 and 8 are excited in the backward direction (-Z direction) with ports-7, 11, and 16, respectively. Moreover, the electrical path length (D1 and D2) between the cDRA is 0.6k and 0.85kat 5.6 GHz as shown in Fig. 6(d). So, together with polarization, pattern diversity, as well as separation between the antenna elements, provides superior isolation between ports- (1, 7); (1, 11) and (1, 16). As a result, the proposed antenna achieves more than 30 dB of isolation between these ports. In this way, all the ports achieve improved isolation without using any extra structure or decoupling network. Table 2 shows different isolation improvement techniques between port-1 and all other ports. From Table 2, it can be observed that a single isolation technique provides only 20 dB of isolation between the ports. But the isolation is improved significantly by 6–7 dB when two isolation improvement techniques are applied simultaneously between the ports. As a result, more than 25 dB of isolation is achieved between the ports. The extreme case when three isolation improvement techniques are approached simultaneously between the ports, and then we have achieved massive improvement in isolation. The isolation between the ports goes beyond 30 dB. In this way, the multiple isolation improvement techniques applied between the ports to improve diversity between the ports. Pol Div-Polarization Diversity, Patt Div- Pattern Diversity, Sep_DRA- Separation between DRA
Table 2 Various isolation techniques between different ports. Ports
Isolation technique
Minimum Isolation Values (dB)
Ports-1,2 Ports-1,3 Ports-1,4 Ports- 1,5 Ports-1, 6 Ports-1,7 Ports-1,8 Ports-1,9 Ports-1,10 Ports-1,11 Ports-1,12 Ports-1,13 Ports-1,14 Ports-1,15 Ports-1,16
Pol Div Patt Div Pol Div + Patt Div Pol Div + Sep_DRA Sep_DRA Pol Div + Patt Div + Sep_DRA Patt Div + Sep_DRA Pol Div + Sep_DRA Sep_DRA Pol Div + Patt Div + Sep_DRA Patt Div + Sep_DRA Sep_DRA Pol Div + Sep_DRA Patt Div + Sep_DRA Pol Div + Patt Div + Sep_DRA
17 19 26 25 21 30 27 26 21 32 25 20 26 27 32
The resonant frequency of the proposed radiator due to generation of HE11d mode is calculated using the following relation [16],
" 2 # 6:321c r r pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:27 þ 0:36 þ 0:02 fr ¼ 2Heff 2Heff 2pr er;eff þ 2
ð1Þ
The effective dielectric constant ðer;eff Þ and effective height (Heff) are calculated using the following formulas [3],
Heff
er;eff ¼
HU
eDRA
ð2Þ
þ eHS
sub
Heff ¼ HU þ HS
ð3Þ
where HU, r (=DU/2) is the height and radius of the top cDR, respectively. Hs and c are the height of the substrate and velocity of light, respectively. With the help of Eqs. (1)–(3), the resonant frequency is calculated as 5.75 GHz, which is closer to the simulated resonant frequency. 4. Experimental results The experimental design antenna prototype is displayed in Fig. 7. Fig. 8 displays the measured |Sii| and |Sij| variation with different the ports, where ‘i’ and ‘j’ is the port number. Fig. 8(a) and (b) shows the reflection coefficient for each port, and the isolation variation is shown in Fig. 8(c) and (d). It can be seen from Fig. 8(a) and (b) that each port covers 5.4–5.9 GHz band. The isolation values are better than 17 dB for all the ports with respect to port-1. These parameters were measured with the help of KEYSIGHT N5221A PNA. During reflection coefficient measurement, one port is connected to PNA, and all other ports are attached to 50 X to minimize the reflection from all other ports. Slightly mismatch observed between experimental and simulated outcomes, which occur because of connector mismatch or use of glue or fabrication tolerances. The simulated and measured 2D far-field patterns of the proposed multi-port radiator with port-1 and port-3 are shown in Fig. 9. Due to the symmetric orientation of cDRs, the far-field patterns for port-1 and 3 are discussed only. Fig. 9 reveals that broadside radiation characteristics (h = 00, / = 00) have been observed from port-1 in XZ- and YZ- plane. Similarly, the opposite of broadside radiation pattern characteristics (h = 1800, / = 00) is observed from port-3. As a result, oppositely oriented patterns are observed from port-1 and port-3, which provides two-directional pattern diversity of the proposed multi-port antenna. Here, two different radiation patterns of the proposed antenna are separated spatially to achieve improved field correlation between the antenna elements. Peak gain of the proposed MIMO radiator for ports- 1, 3, is shown in Fig. 10. It reveals that more than 5 dB gain is observed from port-1 and port-3, respectively. The envelope correlation coefficient (ECC) of the proposed MIMO antenna is computed by using the far-field based formula. ECC between the i-th and j-th antenna elements can be calculated by using the following relation [19],
h i 2 RR ! ! F i ðh; /Þ F j ðh; /Þ dX 4p qe ¼ RR ! 2 2 RR ! F i ðh; /Þ dX F j ðh; /Þ dX 4p
ð4Þ
4p
! ! where F i ðh; /Þ and F j ðh; /Þ presents the radiation pattern of the antenna when port-i and port-j is excited respectively. * represents hermitian product operator and X presents solid angle. Fig. 11(a) and (b) shows the ECC of the proposed radiator between different
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Fig. 7. Photograph of the fabricated antenna structure (a) top view, (b) side view and (c) bottom view.
Fig. 8. Measured scattering parameters of the proposed radiator; (a) Reflection coefficient for ports-1 to 8; (b) Reflection coefficient for ports- 9 to 16; (c) Isolation variation (Ports-2 to 8 with respect to port-1) and (d) Isolation variation (Ports-9 to 16 with respect to port-1.
ports. It reveals that the ECC between the ports lies well below the tolerable limit of 0.5 [19]. Diversity gain (DG) of the proposed MIMO antenna is displayed in Fig. 11(c) and (d). It is computed by using the formula [20].
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi Diversity GainðDGÞ ¼ 10 1 qe
ð5Þ
It can be seen that the DG of the MIMO antenna between different ports closer to 10, which guarantees improved diversity performance of the proposed MIMO antenna [19]. Mean effective gain (MEG) defines as a ratio between received power by multiport antenna and power received by the isotropic antenna. In an isotropic environment, the MEG of the proposed multiport antenna is calculated by using the following relation [20].
h 2 i MEGi ¼ 0:5 1 jSii j2 Sij
ð6Þ
where Sii represents the reflection coefficient of the particular port, and Sij shows the isolation between port-i and port-j. The MEG of the proposed antenna is calculated for each port and displayed in Fig. 12. To achieve optimum MIMO performance, the ratio between MEGs obtained from different ports should be less than three [15]. The proposed antenna delivers improved MIMO performance since the value of MEGs obtained from different ports is almost equal at the working frequency band, which guarantees that their ratio is also equal to one. Channel capacity loss (CCL) shows the maximum data rate up to which the massage can be constantly transmitted with negligible loss. The CCL of the proposed antenna can be evaluated by utilizing the following relation from Eqs. (7)–(10) presented in [21].
C loss ¼ log2 bR
ð7Þ
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Fig. 9. Simulated and measured radiation patterns of the proposed multi-port antenna (a) port-1; (b) port-3.
Where,
bii ¼ 1
bij ¼
Fig. 10. Gain of the proposed multi-port radiator.
2
b11 6 . R 6 b ¼ 4 .. b16;1
3 b1;16 .. .. 7 7 . . 5 b16;16
ð8Þ
X16 S S n¼1 in ni
X16 S S n¼1 in nj
ð9Þ
ð10Þ
For i, j = 1, 2, 3, 4. . .. . ...up to 16 The simulated and measured CCL plot of the proposed antenna is presented in Fig. 13. It reveals that the CCL values are less than the recommended value (0.5 bits/sec/Hz) at the working frequency band [21]. Table 3 shows a performance comparison study of the proposed radiator with other multi-port radiators in terms of their size, number of DR elements, number of ports, covered band and isolation level. Table 3 reveals that the proposed antenna consists the number of ports with compact in size as compared to previous work, also delivered improved port isolation level, which is the main requirement of the future wireless access point. As per the author’s knowledge, this is the most compact DRA based MIMO antenna that supports 16- ports.
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Fig. 11. ECC and Diversity Gain of the proposed multi-port radiator (a) ECC from Ports- 2 to 8 with respect to port-1; (b) ECC from Ports- 9 to 16 with respect to port-1 (c) DG from Ports 2 to 8 with respect to port-1; (d) DG from Ports- 9 to 16 with respect to port-1.
Fig. 12. MEG of the proposed antenna. Fig. 13. capacity loss of the proposed antenna.
5. Conclusion This article has given the explanation for the design and analysis of an eight-element sixteen port cDRA based MIMO antenna. Each top cDRA is excited by two orthogonally placed CPW feedlines, whereas two orthogonally fed microstrip lines feed each bottom cDRA. Three different isolation improvement approaches i.e
polarization diversity, pattern diversity, and separation between DRA, are applied within the antenna structure to facilitate improved isolation between the ports. Measured radiation pattern shows that the proposed radiator offer dual-directional pattern diversity. The proposed structure consists of more no. of antenna ports as compared to DR elements, which makes it a compact
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Table 3 Performance evaluation of the proposed radiator with other published multi-port radiators.
[4] [5] [6] [7] [8] [9] This Work
Antenna Size (mm3)
Isolation (dB)
Covered Bands (GHz)
No of DR elements
No. of Ports
Isolation Techniques
160 160 14.8 56.6 31.5 14.09 100 80 13.56 100 100 17.62 80 80 7.6 50 50 14.6 90 80 14.6
15 20 12 20 20 25 17
2.38–2.5 and 5.7–6 9.12–9.84 2.6–2.74 3.78–4.07 2.56–2.64 3.1–3.6 5.4–6.0
8 1 2 1 1 1 8
8 3 6 2 2 2 16
Pattern Polarization + Pattern polarization Pattern Pattern Polarization Polarization+Pattern + Separation between DRA
structure. The proposed radiator is appropriate for 5.8 GHz WLAN standards. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.aeue.2020.153193. References [1] Zhou Q, Dai H. Joint antenna selection and link adaptation for MIMO systems. IEEE Trans Vehicular Techn. 2006;55:243–55. [2] Sharawi MS. Printed MIMO antenna systems: performance metrics, implementations and challenges. Forum Electromagn Res Methods Appl Technol 2014;1:1–11. [3] Petosa A. Dielectric resonator antenna handbook. Norwood, MA, USA: Artech House; 2007. [4] Sharawi MS, Podilchak SK, Khan MU, et al. Dual frequency DRA based MIMO antenna system for wireless access point. IET Microw Antennas Propag 2017;11:1174–82. [5] Abdalrazik A, Hameed ASA, Rahman AB. A three port MIMO dielectric resonator antenna using decoupled modes. IEEE Antenn Wirel Propag Lett 2017;16:3104–7. [6] Tian R, Plicanic V, Lau BK, Ying Z. A compact six-port dielectric resonator antenna array: MIMO channel measurements and performance analysis. IEEE Trans Antenn Propag 2010;58:1369–79. [7] Zou L, Abbott D, Fumeaux C. Omnidirectional cylindrical dielectric resonator antenna with dual polarization. IEEE Antenn Wirel Propag Lett 2012;11:515–8.
[8] Nasir J, Jamaluddin MH, Khalily M, Kamarudin MR, Ullah I. Design of an MIMO dielectric resonator antenna for 4G applications. Wireless Pers Commun 2016;88:525–36. [9] Das G, Sharma A, Gangwar RK. Dual port aperture coupled MIMO cylindrical dielectric resonator antenna with high isolation for WiMAX application. Int J RF Microw Comp Aided Engg 2016;27:1–8. [10] Das G, Sharma A, Gangwar RK, Sharawi MS. Compact back-to-back DRA-based four-port MIMO antenna system with bi-directional diversity. IET Electron lett 2018;54:884–6. [11] Goldsmith A, Jafar SA, Jindal N, et al. Capacity limits of MIMO channels. IEEE J Sel Areas Commun 2003;21:684–702. [12] Cisco Aironet Series 1700/2700/3700 access points. Available at http:// www.cisco.com. [13] Sharawi MS. Current misuses and future prospects for printed multiple-inputmultiple-output antenna systems. IEEE Antenn Propag Magzaine 2017;59:162–70. [14] Gao Y, Feng Z, Ahang Li. Compact CPW-fed dielectric resonator antenna With dual polarization. IEEE Antenn Wirel Propag Lett 2011;10:544–7. [15] Khan AA, Khan R, Aqeel S, Nasir J, Saleem J, Owais MK. Design of a dual-band MIMO dielectric resonator antenna with high port isolation for WiMAX and WLAN applications. Int J RF Microw Comp Aided Engg 2015;25:1–11. [16] Mongia RK, Bhartia P. Dielectric resonator antennas-a review and general design relations for resonant frequency and bandwidth. Int J RF Microw Comp Aided Engg 1994;4:230–47. [17] Srivastava G, Mohan A. Compact MIMO slot antenna for UWB applications. IEEE Antenn Wirel Propag Lett 2016;15:1057–60. [18] Dwivedi AK, Sharma A, Singh AK, Singh V. Quad port ring Dielectric Resonator based MIMO radiator with polarization and space diversity. Microw Optic Techn Lett 2020:1–12. https://doi.org/10.1002/mop.32329. [19] Sharawi MS. Printed MIMO antenna engineering. Norwood, MA, USA: Artech House; 2014. [20] Kumar A, Ansari AQ, Kanaujia BK, Kishor J, Kumar S. An ultra compact two port UWB MIMO antenna with dual band notched characteristics. AEU-Int J Electron Commun 2020;114:1–12. [21] Tiwari RN, Singh P, Kanaujia BK, Srivastava K. Neutralization technique based two and four port high isolation MIMO antennas for UWB communication. AEU-Int J Electron Commun 2019;110:1–10.
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International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Regular paper
Dielectric resonator based multiport antenna system with multi-diversity and built-in decoupling mechanism Gourab Das, Nikesh Kumar Sahu, Ravi Kumar Gangwar ⇑ Department of Electronics Engineering, Indian Institute of Technology (ISM) Dhanbad, Jharkhand 826004, India
a r t i c l e
i n f o
Article history: Received 19 February 2020 Accepted 27 March 2020
Keywords: Multiple-input-multiple-output (MIMO) Dielectric resonator antenna (DRA) Diversity techniques Envelope correlation coefficient (ECC)
a b s t r a c t In this article, an eight-element, sixteen port dielectric resonator (DR) based double sided multi-port antenna is presented, which supports the unique dual-directional pattern diversity feature. The orientation of the double-sided dielectric resonator antenna (DRA) is one of the novel features of the proposed antenna. Four cylindrical dielectric resonator antennas (cDRAs) are placed at the top of the substrate, and each top cDRA is excited with the help of two orthogonally placed coplanar waveguide (CPW) fed conformal microstrip lines. In the same manner, the other four cDRAs are placed at the bottom of the substrate, and each cDRA is fed via conformal strip lines. The proposed antenna works in between 5.4 GHz and 6.0 GHz. Several isolation techniques (polarization diversity, pattern diversity, and separation between the antenna elements) within the antenna elements are approached to improve the antenna performance. As a result, without using any isolation improvement technique, the proposed structure achieves 17 dB of isolation between the ports, which makes the proposed radiator unique. The similarity in simulated and experimental outcomes confirms that the designed 16-port multi-port antenna system is suitable for Wireless LAN applications. Ó 2020 Elsevier GmbH. All rights reserved.
1. Introduction Modern cellular communication systems require high data rates with improved system capacity and reliability. This requirement enforces antenna engineers to design and develop multi-port antenna systems. Multi-port antenna systems are designed by placing more than one radiator on a single substrate. Multiport MIMO technology generally uses diversity techniques to improve the reliability of any communication system by mitigating the multipath fading problem. To improve spectral efficiency, the main aim is to spread the total transmitted power over the radiators. Thus, MIMO antennas contribute to the enhancement of the reliability and data rate of the communication system without losing an additional transmitted spectrum or power [1]. In MIMO based radiating structure, the correlation between the fields obtained from multiple ports should be minimum. But, it is problematic to adjust many radiators in a single antenna system with maintaining high isolation since the radiation pattern of different radiators can affect each other as well as the MIMO channel characteristics. To improve the port isolation level, one can use
⇑ Corresponding author. E-mail addresses: [email protected] (G. Das), nikesh.sahu100@gmail. com (N.K. Sahu), [email protected] (R.K. Gangwar). https://doi.org/10.1016/j.aeue.2020.153193 1434-8411/Ó 2020 Elsevier GmbH. All rights reserved.
different isolation techniques such as reflectors/defected ground structure (DGS)/parasitic elements, but it is at the cost of radiation characteristics [2]. So, isolation improvement without any extra decoupling structure gives good matching efficiencies and radiation characteristics. On the other hand, dielectric antennas have gained extensive recognition in cellular communication, since it offers numerous advantages like high radiation efficiency, no conductor loss, improved gain, ease of excitation among others [3]. Therefore, at microwave and millimeter-wave frequencies (metallic losses are very high), dielectric based radiators are widely used [4–9]. In [4], the design and analysis of 8-port based MIMO cDRA for 2.4/5.8 GHz WLAN application were explained. For beam tilting, a reflector was used to achieve pattern diversity. The radiator, as mentioned above, takes over a huge volume of 160 160 14.8 mm3. A rectangular DRA based three ports MIMO antenna with a dimension of 56.6 31.5 14.09 mm3 was described in [5]. The triple port produced three decoupled degenerate modes in the DRA to achieve improved isolation between the ports. Probe fed based six-port two-element MIMO DRA was discussed in [6]. Each DRA was excited by three different ports. The occupied area of the proposed antenna was 100 80 13.56 mm3. It used spatial and polarization diversity to achieve low mutual coupling between the ports. An annular-shaped cDRA supporting dual-port pattern
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diversity was presented in [7]. The pattern diversity was accomplished by stimulating the two degenerate orthogonal modes (TE01d and TM01d) in the annular cDRA. The size of the realized radiator was 100 100 17.62 mm3. In [8], a dual-port rectangular DRA with a dimension of 80 80 7.6 mm3 was proposed for 2.6 GHz LTE band. The isolation was improved through pattern diversity, by etching two rectangular slits from the ground plane. In [9], a dual-port DRA based MIMO antenna with an occupied area of 50 50 14.6 mm3 was discussed. Polarization diversity was achieved by stimulating orthogonal modes in the DRA. A compact back to back four-port MIMO DRA was discussed in [10]. The antenna used polarization and pattern diversity for isolation improvement between the ports. In literature, most of the DRA based multi-port radiators consisted of 2-ports. However, such type of antenna cannot improve the quality of the transmission link as the channel capacity has a linear relationship with the number of spatial streams generated by each antenna ports [11]. Due to these reasons, recently, some of the DRA based MIMO antennas have been reported, which consist of more than two ports [4–6]. However, most of these antennas are large or having poor MIMO performance. In this paper, a compact double-sided DRA based sixteen-port MIMO radiator is presented for wireless access point applications. The compactness is achieved by placing back to back DRA orientation since this type of arrangement provides effective utilization of the ground plane. Four cDRA are positioned at the top of the substrate and excite in the forward direction. Another four cDRA are placed at the lower side of the substrate and radiated in the backward direction. The antenna consists of sixteen ports since each cDRA is stimulated with the help of two ports.
Additionally, for the very first time, three distinct different isolation techniques i.e polarization diversity, pattern diversity, and separation between the DR, are used within a single antenna system for isolation improvement. As a result, this radiating structure attains low mutual coupling (better than 17 dB) within the operating band without using any extra structure. Also, the proposed antenna provides two oppositely oriented radiation patterns in space that means higher chances of having independent channels, thus deliver higher channel capacities as well as low field correlation. The proposed radiator is easy to construct, and the total volume of the realized radiator is 90 80 14.6 mm3. The proposed antenna also analyzes how the combination of two or more diversity techniques at a time improves isolation in detail. Different MIMO performance parameters like envelope correlation coefficient (ECC), diversity gain (DG), mean effective gain (MEG), and channel capacity loss (CCL) are also analyzed. More significantly, the proposed MIMO antenna system described in this paper is a low profile for a commercially available wireless access point system, which enables fourth-generation communication systems [12]. Section-II and III of the presented paper show the geometry of the antenna and analysis of the proposed antenna, respectively. The results and discussion of the proposed antenna and conclusion are described in section-IV and V, respectively.
2. Formation of the proposed radiator The geometry of the projected 16-port dielectric resonatorbased radiator is displayed in Fig. 1. From Fig. 1(a), it can be perceived that four alumina ceramic-based cDRAs (DR-1, 3, 5 and 7)
Fig. 1. Geometry of the proposed MIMO antenna (a) Top-view (b) bottom-view; (c) Transparent side view; (d) side view of DR-1 and 2.
G. Das et al. / Int. J. Electron. Commun. (AEÜ) 119 (2020) 153193 Table 1 Optimized design parameters. Symbol
Dimension (mm)
Symbol
Dimension (mm)
LS WS Hs L1 = L2 W1 = W2 P1
90 80 1.6 8 2 0.25
H1 H2 LX LY DU = DL HU = HL
6.5 6.0 30 20 14 6.5
(edra = 9.8) are positioned at the top of the FR4 substrate (esub = 4.4). Each of the top cDRA is excited with the help of two orthogonally fed CPW fed conformal strip lines. On the other hand, another four ceramic material based cDRAs (DR-2, 4, 6 and 8) are placed at the bottom of the substrate. cDRA, located at the bottom of the substrate, is excited by two microstrip line fed conformal strip lines as shown in Fig. 1(b). To accommodate Ports-1, 5, 9 and 13, a portion of the substrate (LX LY) has been removed. As a result, the top of the substrate contains a total of eight ports. The top of the substrate consists of 4 cDRA with eight different ports; similarly, the bottom of the substrate contains 4- cDRAs along with 8- individual ports that make the proposed antenna as 16- port MIMO antenna. Different optimized design parameters are shown in Table 1. It should be noted that the length (LX and LY) of the rectangular removed portion is not equal since such configuration offers equal length for all the feedings and the connected ground plane between the different antenna elements. In the MIMO system, the connected ground plane is important to achieve improved diversity performance [13].
3. Analysis of the proposed radiator The design and analysis of the proposed radiating structure have been performed with the assistance of Ansys HFSS EM simulation software. Figs. 2 and 3 show the reflection coefficient of each port and isolation between the ports of the given multi-port radiator for various ports, respectively. Fig. 2 reveals that all the ports can able to cover the band 5.8 GHz WLAN (5.725–5.875 GHz) band. There is a minor difference between the reflection coefficients which occur due to the utilization of two different feeding mechanisms [14–15]. Fig. 3 shows the isolation variation for ports-2 to 16 with respect to port-1. From Fig. 3, it can be observed that the isolation between the ports is better than 15 dB for all the cases. In the MIMO antenna system, the isolation is achieved between the ports by imposing different diversity techniques. In the proposed MIMO
3
antenna, three different isolation improvement techniques (polarization diversity, pattern diversity, and separation between the elements), as well as their combination, are used to achieve improved diversity between the ports. Different isolation improvement techniques between the ports are discussed below: 3.1. Polarization diversity Polarization diversity is established by exciting orthogonal mode in the cDRA. This approach is applied in DR-1 due to ports1 and 2. DR-1 supports dual hybrid modes i.e. HEx11d and HEy11d modes because of ports- 1 and 2, respectively. Since, HEx11d and HEy11d modes are decoupled orthogonal mode (via polarization difference), so polarization diversity plays a crucial role in achieving improved isolation between port-1 and 2. Fig. 4(a) shows E-field lines orientation in DR-1 due to port-1 and port-2. It reveals that orthogonal modes ðHEx11d and HEy11d ) are generated in DR-1. Here, HEx11d and HEy11d denotes variation of field lines along the X-axis and Y-axis, respectively. As a result, the proposed antenna achieves 17 dB of isolation between ports-1 and 2. 3.2. Pattern diversity Pattern diversity is achieved by exciting the cDRA in a different direction to realize a spatially de-correlated beam pattern. From Fig. 1(c), it can be perceived that DR-2 is positioned at the bottom side of the substrate, just below of the DR-1. DR-2 is excited by ports- 3 and 4. The exclusive feature of HE11d mode is that it always radiates in the broadside direction [16]. In this antenna structure, this elite feature of HE11d mode has been reserved in different directions with the assistance of different port to improve port as well as field isolation. Fig. 4(b) shows the 3D polar plot of the proposed antenna for Ports-1 and 3. Fig. 4(b) reveals that DR-1 is excited in the broadside (+Z) direction, whereas DR-2 is excited in opposite of broadside (-Z) direction with the help of port-3 and 4. So, DR-1 and DR-2 are excited in a complementary direction, which creates pattern diversity in two opposite directions. The radiation beam of DR-1 and DR-2 is separated spatially. As a result, we get improved isolation (S13) values (19 dB) between ports- (1, 3) in the working frequency range. So, the isolation between ports- (1, 3) has been achieved through pattern diversity. 3.3. Separation between the DR elements It is a well-known fact that mutual coupling among the antenna elements is high because of field interaction. So, this field interac-
Fig. 2. Reflection Coefficient of the proposed antenna (a) Reflection coefficient (Ports-1 to 8); (b) Reflection coefficient (Ports- 9 to 16).
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Fig. 3. Isolation response of the proposed MIMO antenna (a) Isolation variation (Ports-2 to 8 with respect to port-1) and (b) Isolation variation (Ports-9 to 16 with respect to port-1).
Port-1
Port-2
(a) Port-1
Port-3
(b) Fig. 4. E-field distribution and 3D-polar plot of the proposed antenna (a) E-field lines for Port-1 and port-2; (b) polar plot for port-1 and port-3.
tion can be minimized by creating a large spacing between them. Fig. 5 shows the minimum effective electrical path length between DR-1 and 3. It reveals that DR-1 and 3 are symmetric with respect to port-1 and port-6, and the spacing (D1) between port-1 and port-6 is 0.6k at 5.6 GHz. So, the spacing between DR elements plays an important role in isolation improvement between ports1 and 6. DR-7 is excited with the help of ports- 13 and 14. Fig. 5 also shows the effective distance between DR-1 and DR-7. Fig. 5 reveals that the effective electrical path length between DR-1 and DR-7 (D2) is also 0.85k at 5.6 GHz. The mutual coupling depends on the distance between the antenna elements. As the separation increases, the isolation level also improves [17,18]. So, the rectangular portion (LX LY), which is removed from the substrate, is used not only to accommodate the four ports. It also creates a large spacing between DR-1 and DR-7. This made the proposed antenna to achieve high isolation (better than 20 dB) between the port-1 and port-13 which is shown in Fig. 3(b).
3.4. Combination of different isolation techniques A combination of isolation improvement techniques means out of three isolation techniques, two or three isolation techniques are approached simultaneously. The advantage of the combined isolation improvement technique is that it can able to improve diversity performance significantly as compared to a single isolation improvement technique. Four cases of combination of isolation techniques are discussed here. They are given below: (i) Case-I (Combination of polarization and pattern diversity) (ii) Case-II (Combination of polarization diversity and separation between the cDRA) (iii) Case-III (Combination of pattern diversity and separation between the cDRA) (iv) Case-IV (Combination of polarization diversity, pattern diversity and separation between the cDRA)
G. Das et al. / Int. J. Electron. Commun. (AEÜ) 119 (2020) 153193
Fig. 5. Separation between DR (1 and 3) and DR (1 and 7);
(i) Case-I: DR-1 and DR-2 are placed at opposite sides of the substrate, as shown in Fig. 6(a). The direction of maximum radiation of DR-1 and 2 is also shown in Fig. 6(a). It can be observed that DR-1 is radiated in the forward direction due to port-1, and DR-2 is
5
radiated in the backward direction due to port-4. So, pattern diversity exhibit between ports-1 and 4. On the other hand, polarization diversity is achieved, since orthogonal modes are generated in DR1 and DR-2 due to port-1 and port-4, respectively. As a result, the proposed antenna achieved more than 25 dB of isolation between these ports. Only polarization diversity (between ports-1 and 2) or pattern diversity (between ports-1 and 3) offers less than 20 dB of isolation between the ports, which we have already discussed earlier. But, the combination of diversity techniques offers 7–8 dB of isolation improvement between the ports. (ii) Case- II: It can be observed from Fig. 6(b) that ports-5 and 9 are placed orthogonally with respect to port-1. The direction of Efield in the respective ports is also shown in Fig. 6(b). It reveals that the orthogonal modal pattern is generated in DR-1 and 3 due to ports-1 and 5, respectively. Similarly, the orthogonality principle is also established in DR-1 and 5 with ports-1 and 9, respectively. So, polarization diversity (via polarization difference) plays an important role in improving isolation between ports- (1, 5) and ports- (1, 9), since decoupled orthogonal modes are generated in the cDRA which is shown in Fig. 6(b). Fig. 6(b) also shows the minimum electrical distance between DR- (1, 3) and DR- (1, 5). The minimum electrical distance (D1) between DR-1 and DR-3/5 is 0.6k at 5.6 GHz. So, polarization, as well as the separation between the cDR, plays a crucial role in achieving improved port isolation (21 dB) between ports- (1, 5) and ports- (1, 9). It can also be
Fig. 6. Combination of isolation techniques between the ports (a) polarization and pattern diversity; (b) polarization and separation between the DRA; (c) pattern and separation between the DRA and (d) polarization diversity, pattern diversity and separation between the DRA.
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observed in Fig. 6(b) that DR-1 and DR-7 are placed orthogonally with respect to port-1 and port-14. The minimum electrical path distance between DR-1 and DR-7 (D2) is 0.85k at 5.6 GHz. So, the combination of polarization diversity and separation between the antenna elements is utilized to achieve high isolation (better than 28 dB) between port-1 and port-14. The isolation level is better in the case of ports- (1, 14) than the ports- (1, 5) or ports- (1, 9), since the D2 value is high compared to D1. (iii) Case-III: It can be observed from Fig. 6(c) that DR-1 and DR(4, 6 and 8) are located at opposite sides of the substrate. The direction of radiation of these DRs is also shown in Fig. 6(c). It reveals that DR-1 is excited + Z direction for port-1 but, DR- 4, 6 and 8 are excited in –Z direction due to ports- 8, 12, and 15, respectively. So, their radiation patterns are separated spatially. Also, the distance (D1) between DR-1 and DR- (4, 6) is 0.6k at 5.6 GHz. On the other hand, the distance (D2) between DR-1 and DR-8 is 0.85k at 5.6 GHz. So, together pattern diversity and separation between the antenna elements provide high isolation (better than 25 dB) between ports- (1, 8), ports- (1, 12), and ports- (1, 15). (iv) Case-IV: It can be observed from Fig. 6(d) that ports- (7, 11 and 16) are positioned orthogonally with respect to port-1. So, polarization diversity exists in between them. Fig. 6(d) also shows that DR-1 excited in the forward direction (+Z-direction) with port1 but, DR- 4, 6 and 8 are excited in the backward direction (-Z direction) with ports-7, 11, and 16, respectively. Moreover, the electrical path length (D1 and D2) between the cDRA is 0.6k and 0.85kat 5.6 GHz as shown in Fig. 6(d). So, together with polarization, pattern diversity, as well as separation between the antenna elements, provides superior isolation between ports- (1, 7); (1, 11) and (1, 16). As a result, the proposed antenna achieves more than 30 dB of isolation between these ports. In this way, all the ports achieve improved isolation without using any extra structure or decoupling network. Table 2 shows different isolation improvement techniques between port-1 and all other ports. From Table 2, it can be observed that a single isolation technique provides only 20 dB of isolation between the ports. But the isolation is improved significantly by 6–7 dB when two isolation improvement techniques are applied simultaneously between the ports. As a result, more than 25 dB of isolation is achieved between the ports. The extreme case when three isolation improvement techniques are approached simultaneously between the ports, and then we have achieved massive improvement in isolation. The isolation between the ports goes beyond 30 dB. In this way, the multiple isolation improvement techniques applied between the ports to improve diversity between the ports. Pol Div-Polarization Diversity, Patt Div- Pattern Diversity, Sep_DRA- Separation between DRA
Table 2 Various isolation techniques between different ports. Ports
Isolation technique
Minimum Isolation Values (dB)
Ports-1,2 Ports-1,3 Ports-1,4 Ports- 1,5 Ports-1, 6 Ports-1,7 Ports-1,8 Ports-1,9 Ports-1,10 Ports-1,11 Ports-1,12 Ports-1,13 Ports-1,14 Ports-1,15 Ports-1,16
Pol Div Patt Div Pol Div + Patt Div Pol Div + Sep_DRA Sep_DRA Pol Div + Patt Div + Sep_DRA Patt Div + Sep_DRA Pol Div + Sep_DRA Sep_DRA Pol Div + Patt Div + Sep_DRA Patt Div + Sep_DRA Sep_DRA Pol Div + Sep_DRA Patt Div + Sep_DRA Pol Div + Patt Div + Sep_DRA
17 19 26 25 21 30 27 26 21 32 25 20 26 27 32
The resonant frequency of the proposed radiator due to generation of HE11d mode is calculated using the following relation [16],
" 2 # 6:321c r r pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:27 þ 0:36 þ 0:02 fr ¼ 2Heff 2Heff 2pr er;eff þ 2
ð1Þ
The effective dielectric constant ðer;eff Þ and effective height (Heff) are calculated using the following formulas [3],
Heff
er;eff ¼
HU
eDRA
ð2Þ
þ eHS
sub
Heff ¼ HU þ HS
ð3Þ
where HU, r (=DU/2) is the height and radius of the top cDR, respectively. Hs and c are the height of the substrate and velocity of light, respectively. With the help of Eqs. (1)–(3), the resonant frequency is calculated as 5.75 GHz, which is closer to the simulated resonant frequency. 4. Experimental results The experimental design antenna prototype is displayed in Fig. 7. Fig. 8 displays the measured |Sii| and |Sij| variation with different the ports, where ‘i’ and ‘j’ is the port number. Fig. 8(a) and (b) shows the reflection coefficient for each port, and the isolation variation is shown in Fig. 8(c) and (d). It can be seen from Fig. 8(a) and (b) that each port covers 5.4–5.9 GHz band. The isolation values are better than 17 dB for all the ports with respect to port-1. These parameters were measured with the help of KEYSIGHT N5221A PNA. During reflection coefficient measurement, one port is connected to PNA, and all other ports are attached to 50 X to minimize the reflection from all other ports. Slightly mismatch observed between experimental and simulated outcomes, which occur because of connector mismatch or use of glue or fabrication tolerances. The simulated and measured 2D far-field patterns of the proposed multi-port radiator with port-1 and port-3 are shown in Fig. 9. Due to the symmetric orientation of cDRs, the far-field patterns for port-1 and 3 are discussed only. Fig. 9 reveals that broadside radiation characteristics (h = 00, / = 00) have been observed from port-1 in XZ- and YZ- plane. Similarly, the opposite of broadside radiation pattern characteristics (h = 1800, / = 00) is observed from port-3. As a result, oppositely oriented patterns are observed from port-1 and port-3, which provides two-directional pattern diversity of the proposed multi-port antenna. Here, two different radiation patterns of the proposed antenna are separated spatially to achieve improved field correlation between the antenna elements. Peak gain of the proposed MIMO radiator for ports- 1, 3, is shown in Fig. 10. It reveals that more than 5 dB gain is observed from port-1 and port-3, respectively. The envelope correlation coefficient (ECC) of the proposed MIMO antenna is computed by using the far-field based formula. ECC between the i-th and j-th antenna elements can be calculated by using the following relation [19],
h i 2 RR ! ! F i ðh; /Þ F j ðh; /Þ dX 4p qe ¼ RR ! 2 2 RR ! F i ðh; /Þ dX F j ðh; /Þ dX 4p
ð4Þ
4p
! ! where F i ðh; /Þ and F j ðh; /Þ presents the radiation pattern of the antenna when port-i and port-j is excited respectively. * represents hermitian product operator and X presents solid angle. Fig. 11(a) and (b) shows the ECC of the proposed radiator between different
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Fig. 7. Photograph of the fabricated antenna structure (a) top view, (b) side view and (c) bottom view.
Fig. 8. Measured scattering parameters of the proposed radiator; (a) Reflection coefficient for ports-1 to 8; (b) Reflection coefficient for ports- 9 to 16; (c) Isolation variation (Ports-2 to 8 with respect to port-1) and (d) Isolation variation (Ports-9 to 16 with respect to port-1.
ports. It reveals that the ECC between the ports lies well below the tolerable limit of 0.5 [19]. Diversity gain (DG) of the proposed MIMO antenna is displayed in Fig. 11(c) and (d). It is computed by using the formula [20].
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi Diversity GainðDGÞ ¼ 10 1 qe
ð5Þ
It can be seen that the DG of the MIMO antenna between different ports closer to 10, which guarantees improved diversity performance of the proposed MIMO antenna [19]. Mean effective gain (MEG) defines as a ratio between received power by multiport antenna and power received by the isotropic antenna. In an isotropic environment, the MEG of the proposed multiport antenna is calculated by using the following relation [20].
h 2 i MEGi ¼ 0:5 1 jSii j2 Sij
ð6Þ
where Sii represents the reflection coefficient of the particular port, and Sij shows the isolation between port-i and port-j. The MEG of the proposed antenna is calculated for each port and displayed in Fig. 12. To achieve optimum MIMO performance, the ratio between MEGs obtained from different ports should be less than three [15]. The proposed antenna delivers improved MIMO performance since the value of MEGs obtained from different ports is almost equal at the working frequency band, which guarantees that their ratio is also equal to one. Channel capacity loss (CCL) shows the maximum data rate up to which the massage can be constantly transmitted with negligible loss. The CCL of the proposed antenna can be evaluated by utilizing the following relation from Eqs. (7)–(10) presented in [21].
C loss ¼ log2 bR
ð7Þ
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Fig. 9. Simulated and measured radiation patterns of the proposed multi-port antenna (a) port-1; (b) port-3.
Where,
bii ¼ 1
bij ¼
Fig. 10. Gain of the proposed multi-port radiator.
2
b11 6 . R 6 b ¼ 4 .. b16;1
3 b1;16 .. .. 7 7 . . 5 b16;16
ð8Þ
X16 S S n¼1 in ni
X16 S S n¼1 in nj
ð9Þ
ð10Þ
For i, j = 1, 2, 3, 4. . .. . ...up to 16 The simulated and measured CCL plot of the proposed antenna is presented in Fig. 13. It reveals that the CCL values are less than the recommended value (0.5 bits/sec/Hz) at the working frequency band [21]. Table 3 shows a performance comparison study of the proposed radiator with other multi-port radiators in terms of their size, number of DR elements, number of ports, covered band and isolation level. Table 3 reveals that the proposed antenna consists the number of ports with compact in size as compared to previous work, also delivered improved port isolation level, which is the main requirement of the future wireless access point. As per the author’s knowledge, this is the most compact DRA based MIMO antenna that supports 16- ports.
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Fig. 11. ECC and Diversity Gain of the proposed multi-port radiator (a) ECC from Ports- 2 to 8 with respect to port-1; (b) ECC from Ports- 9 to 16 with respect to port-1 (c) DG from Ports 2 to 8 with respect to port-1; (d) DG from Ports- 9 to 16 with respect to port-1.
Fig. 12. MEG of the proposed antenna. Fig. 13. capacity loss of the proposed antenna.
5. Conclusion This article has given the explanation for the design and analysis of an eight-element sixteen port cDRA based MIMO antenna. Each top cDRA is excited by two orthogonally placed CPW feedlines, whereas two orthogonally fed microstrip lines feed each bottom cDRA. Three different isolation improvement approaches i.e
polarization diversity, pattern diversity, and separation between DRA, are applied within the antenna structure to facilitate improved isolation between the ports. Measured radiation pattern shows that the proposed radiator offer dual-directional pattern diversity. The proposed structure consists of more no. of antenna ports as compared to DR elements, which makes it a compact
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Table 3 Performance evaluation of the proposed radiator with other published multi-port radiators.
[4] [5] [6] [7] [8] [9] This Work
Antenna Size (mm3)
Isolation (dB)
Covered Bands (GHz)
No of DR elements
No. of Ports
Isolation Techniques
160 160 14.8 56.6 31.5 14.09 100 80 13.56 100 100 17.62 80 80 7.6 50 50 14.6 90 80 14.6
15 20 12 20 20 25 17
2.38–2.5 and 5.7–6 9.12–9.84 2.6–2.74 3.78–4.07 2.56–2.64 3.1–3.6 5.4–6.0
8 1 2 1 1 1 8
8 3 6 2 2 2 16
Pattern Polarization + Pattern polarization Pattern Pattern Polarization Polarization+Pattern + Separation between DRA
structure. The proposed radiator is appropriate for 5.8 GHz WLAN standards. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.aeue.2020.153193. References [1] Zhou Q, Dai H. Joint antenna selection and link adaptation for MIMO systems. IEEE Trans Vehicular Techn. 2006;55:243–55. [2] Sharawi MS. Printed MIMO antenna systems: performance metrics, implementations and challenges. Forum Electromagn Res Methods Appl Technol 2014;1:1–11. [3] Petosa A. Dielectric resonator antenna handbook. Norwood, MA, USA: Artech House; 2007. [4] Sharawi MS, Podilchak SK, Khan MU, et al. Dual frequency DRA based MIMO antenna system for wireless access point. IET Microw Antennas Propag 2017;11:1174–82. [5] Abdalrazik A, Hameed ASA, Rahman AB. A three port MIMO dielectric resonator antenna using decoupled modes. IEEE Antenn Wirel Propag Lett 2017;16:3104–7. [6] Tian R, Plicanic V, Lau BK, Ying Z. A compact six-port dielectric resonator antenna array: MIMO channel measurements and performance analysis. IEEE Trans Antenn Propag 2010;58:1369–79. [7] Zou L, Abbott D, Fumeaux C. Omnidirectional cylindrical dielectric resonator antenna with dual polarization. IEEE Antenn Wirel Propag Lett 2012;11:515–8.
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